Non-typhoon Periods Research Articles

Effects of Typhoon Mujigae on the Biogeochemistry and Ecology of a Semi‐Enclosed Bay in the Northern South China Sea

AbstractTyphoons frequently induce algal blooms in the open ocean based on the satellite remote sensing data. However, the effects of typhoons on nearshore areas are not fully clarified due to the inaccessibility of in situ sampled data and the low accuracy of satellite data in the nearshore areas. To understand the biogeochemical and ecological changes in semi‐enclosed bays under the influence of typhoons, physicochemical parameters and isotopic compositions of surface water were measured in Zhanjiang Bay during the super Typhoon Mujigae and nontyphoon periods. The results show that the chlorophyll a (Chl a) concentrations inside and outside the bay during these two periods were considerably different. During typhoon periods, the positive apparent oxygen utilization and unsaturated dissolved oxygen (DO) values suggested significant biological degradation in Zhanjiang Bay, rather than the algal blooms reported by previous studies. In contrast, during nontyphoon periods, particulate organic matter (POM: POC and PN) was mainly generated by in situ phytoplankton production, as indicated by a low POC/Chl a ratio (<99), low C/N ratio, and positive correlations between PN, POC, and Chl a. Isotopic analysis of the particulate N (PN) and nitrate (NO3−) indicated that the 15N‐enriched PN in both periods originally came from sewage‐dissolved inorganic N. These results suggest that the typhoon‐induced decomposition processes, rather than algal blooms, in this semi‐enclosed bay in the northern South China Sea, providing new understanding of the impact of the typhoons on marine biogeochemistry and ecology.

Effects of islands and downslope seafloors on underwater noise in the northern South China Sea during a typhoon

The correlation of ambient noise with wind speed, and the depth dependence of ambient noise are both investigated, where the ocean noise data were recorded by a vertical line array in the northern South China Sea. It is shown that the correlation coefficients increase with increasing hydrophone depth during typhoon periods when the frequency ⩾ 250 Hz, which opposes the generally accepted knowledge that the correlation coefficients of noise level and wind speed decrease with increasing depth during non-typhoon periods. Particularly at frequencies of 250 Hz, 315 Hz and 400 Hz, the correlation coefficients increase by more than 0.05 at depths ranging from 155 m to 875 m. At the three frequencies, the average noise levels also increase with increasing depth during typhoon periods. It is suggested that these differences are attributed to the wind-generated noise in shallow waters and the effect of “downslope enhancement” to sound propagation. During typhoon periods, the surf breaking and surf beat upon the shores and reefs are strengthened, and the source levels are increased. The wind-generated noise in shallow waters interacts with the downslope sea floor, with the noise-depth distribution changed by a “downslope enhancement” effect promoting noise propagation.

Temporal variation of chemical weathering rate, source shifting and relationship with physical erosion in small mountainous rivers, Taiwan

Temporal variation of the chemical weathering rate (CWR) versus physical erosion rate (PER) and the shift of the sources of dissolved solids during rainstorms are explored in two small mountainous rivers, Northern Taiwan. The ionic hydrochemistry of streamwater, rainwater, soil water, and groundwater were collected to quantitatively demonstrate the sources of the dissolved solids and the coupling effect between CWR and PER during typhoon and non-typhoon periods. Results indicate that silicate cation weathering (CWRSC) varies between 33 and 37 t km−2 yr−1, which is ~7 times the global mean, and the annual PER is ~81–166 t km−2 yr−1, similar to the global mean of ~137 t km−2 yr−1. The rainstorm periods only account for 1% of a year but are responsible for 10% of the CWRSC and over 80% of the sediment flux, highlighting the importance of storm events. Silicate cation concentrations present a simple dilution process during rainstorms indicates the high connectivity between hillslopes and streams during rainstorms is established and forces the soil water into streams rapidly. Besides, the coupling between CWRSC and PER becomes less relevant during rainstorms. The coupling between CWRSC and PER is non-linear with flow regime dynamics, and the estimates of CWRSC derived from non-typhoon day are as largely biased as one order of magnitude. High-resolution water chemistry data on the variations in ion production and physical erosion is more valuable than previously thought for characterizing the CWR and PER of active mountain belts.

Changes in environmental flow regimes caused by tropical cyclones in the Western North Pacific

Changes in environmental flow regime due to hydraulic construction projects and variability in precipitation patterns due to climate change over the past several years have increased the stress on ecohydrology, and the indiscreet use of water has had a negative impact on the water environment in natural rivers and hydro structures. Previous studies on typhoon precipitation have simply estimated the intensity, duration, frequency, and magnitude of typhoon precipitations, but the study of the effects on water resource and ecosystem operations by separating typhoon precipitations from warm season precipitations was relatively inadequate. Therefore, this study investigated the characteristics of quantitative hydrological changes related to daily flow data of dam inflows considering the effects of the typhoon. According to the peak flow rate and frequency analysis, there is a distinct increase trend in the southern part of the Korean peninsula. Also, during non-typhoon periods, low flow rates are frequent, but low-flow changes due to typhoons are not significant. The results of this study could be used as a useful resource for establishing scientific measures for the preparation and coordination of responses to the changes in water and ecological environment systems in a changing climate.

Observation of Near-Inertial Oscillations Induced by Energy Transformation during Typhoons

Three typhoon events were selected to examine the impact of energy transformation on near-inertial oscillations (NIOs) using observations from a subsurface mooring, which was deployed at 125° E and 18° N on 26 September 2014 and recovered on 11 January 2016. Almost 16 months of continuous observations were undertaken, and three energetic NIO events were recorded, all generated by passing typhoons. The peak frequencies of these NIOs, 0.91 times of the local inertial frequency f, were all lower than the local inertial frequency f. The estimated vertical group velocities (Cgz) of the three NIO events were 11.9, 7.4, and 23.0 m d−1, and were relatively small compared with observations from other oceans (i.e., 100 m d−1). The directions of the horizontal near-inertial currents changed four or five times between the depths of 40 and 800 m in all three NIO events, implying that typhoons in the northwest Pacific usually generate high-mode NIOs. The NIO currents were further decomposed by performing an empirical orthogonal function (EOF) analysis. The first and second EOF modes dominated the NIOs during each typhoon, accounting for more than 50% of the total variance. The peak frequencies of the first two EOF modes were less than f, but those of the third and fourth modes were higher than f. The frequencies of all the modes during non-typhoon periods were more than f. Our analysis indicates that the relatively small downward group velocity was caused by the frequent direction changes of the near-inertial currents with depth.

Isotopic evidence for the influence of typhoons and submarine canyons on the sourcing and transport behavior of biospheric organic carbon to the deep sea

Export of biospheric organic carbon from land masses to the ocean plays an important role in regulating the global carbon cycle. High-relief islands in the western Pacific are hotspots for such land-to-ocean carbon transport due to frequent floods and active tectonics. Submarine canyon systems serve as a major conduit to convey terrestrial organics into the deep sea, particularly during episodic floods, though the nature of ephemeral sediment transportation through such canyons remains unclear. In this study, we deployed a sediment trap in southwestern Taiwan's Gaoping submarine canyon during summer 2008, during which Typhoon Kalmaegi impacted the study area. We investigated sources of particulate organic carbon and quantified the content of fossil organic carbon (OCf) and biospheric non-fossil carbon (OCnf) during typhoon and non-typhoon periods, based on relations between total organic carbon (TOC), isotopic composition (δ13C, 14C), and nitrogen to carbon ratios (N/C) of newly and previously reported source materials. During typhoons, flooding connected terrestrial rivers to the submarine canyon. Fresh plant debris was not found in the trap except in the hyperpycnal layer, suggesting that only hyperpycnal flow is capable of entraining plant debris, while segregation had occurred during non-hyperpycnal periods. The OCnf components in typhoon flood and trapped samples were likely sourced from aged organics buried in ancient landslides. During non-typhoon periods, the canyon is more connected to the shelf, where waves and tides cause reworking, thus allowing abiotic and biotic processes to generate isotopically uniform and similarly aged OCnf for transport into the canyon. Therefore, extreme events coupled with the submarine canyon system created an efficient method for deep-sea burial of freshly produced organic-rich material. Our results shed light on the ephemeral transport of organics within a submarine canyon system on an active tectonic margin.

Variational Characteristics and Implications of Gaseous Elemental Mercury for Three Continuous Typhoons in China.

Coupling air pollutants with particular meteorological conditions can induce air pollution episodes. To our knowledge, how typhoons influence mercury (Hg) as an extreme weather phenomena has not been reported. Gaseous elemental Hg (GEM) was measured during a time period (from September 16, 2011 to October 9, 2011) that included three typhoons (Haitang, Nesat, and Nalgae) at the Wuzhishan National Atmospheric Background Station. The GEM concentration during these typhoons ranged from 1.81 to 4.73 ng/m(3) (2.97 ± 0.58 ng/m(3)), 1.27 to 4.42 ng/m(3) (2.69 ± 0.83 ng/m(3)), and 1.43 to 2.99 ng/m(3) (2.47 ± 0.32 ng/m(3)), which was higher than for the non-typhoon period (1.14-2.93 ng/m(3), 1.61 ± 0.52 ng/m(3)). Simultaneously, the three typhoon periods exhibited a significant positive correlation between the GEM concentration and wind speed. These results differ from the common belief that lower pollutant concentrations will occur due to a typhoon accelerating pollutant diffusion. Changes in the wind direction and long range pollutant transport from the Chinese mainland can reasonably account for this abnormality. There was a significantly positive correlation between the GEM and SO2, NO x , CO, and O3 levels during the three typhoons periods, which indicates they came from the same sources or areas. A backward trajectory analysis and the concentration weighted field at our monitoring site indicated that clean air masses mainly came from Southeast Asia or the southeast and northeast sea surfaces during non-typhoon periods, while polluted air masses came from the Chinese mainland during the three typhoon periods. The results implied that the increased GEM concentrations in the Wuzhi Mountain were caused by the long-range atmospheric transport of Hg from the Chinese mainland during the typhoon periods. The combustion of coal may be the main emission sources.

Typhoon effects on phytoplankton responses in a semi-closed freshwater ecosystem

Analyses of past typhoons have suggested that global climate change may result in increases in the intensity of these episodic events and that the effects of typhoons on the biogeochemistry of aquatic ecosystems will also be strengthened. We collected a 2-year time series of phytoplankton responses, including chlorophyll-a concentration, primary production and turnover rate in the Fei-Tsui Reservoir, Taiwan, on the basis of five typhoons in 2011 and eight in 2012 (21 weeks affected). We found approximately a two-fold increase in phytoplankton responses during the typhoon period compared with the non-typhoon period. However, there were no consistent correlations between phytoplankton responses and typhoon disturbance and length of typhoon stay. Vertical distributions of phytoplankton responses indicated that the peak values of these responses occurred both during the typhoon periods and during the non-typhoon periods occurring between two typhoons. Moreover, the strongest correlations were found between euphotic depth-averaged phosphate and primary production and turnover rate. Combined effects on phytoplankton responses could explain at least 70% of the variability. The regulation of phytoplankton responses by multiple processes and interactions among factors that operate during each typhoon event may add complexity to the challenge of detecting typhoon-driven mechanisms in such ecosystems.

Impacts of tropical cyclones on hydrochemistry of a subtropical forest

Abstract. Tropical cyclones (typhoons/hurricanes) have major impacts on the biogeochemistry of forest ecosystems, but the stochastic nature and the long intervals between storms means that there are limited data on their effects. We characterised the impacts of 14 typhoons over six years on hydrochemistry of a subtropical forest plantation in Taiwan, a region experiencing frequent typhoons. Typhoons contributed 1/3 of the annual rainfall on average, but ranged from 4 to 55%. The stochastic nature of annual typhoon related precipitation poses a challenge with respect to managing the impacts of these extreme events. This challenge is exacerbated by the fact that typhoon-related rainfall is not significantly correlated with wind velocity, the current focus of weather forecasts. Thus, little advance warning is provided for the hydrological impacts of these storms. The typhoons we studied contributed approximately one third of the annual input and output of most nutrients (except nitrogen) during an average 9.5 day yr−1 period, resulting in nutrient input/output rates an order of magnitude greater than during non-typhoon months. Nitrate output balanced input during the non-typhoon period, but during the typhoon period an average of 10 kg ha−1 yr−1 nitrate was lost. Streamwater chemistry exhibited similarly high variability during typhoon and non-typhoon periods and returned to pre-typhoon levels one to three weeks following each typhoon. The streamwater chemistry appears to be very resilient in response to typhoons, resulting in minimal loss of nutrients.

Carbon and nitrogen isotopic compositions of particulate organic matter and biogeochemical processes in the eutrophic Danshuei Estuary in northern Taiwan

The Danshuei Estuary is distinctive for the relatively short residence time (1-2 d) of its estuarine water and the very high concentration of ammonia, which is the dominant species of dissolved inorganic nitrogen in the estuary, except near the river mouth. These characteristics make the dynamics of nitrogen cycling distinctively different from previously studied estuaries and result in unusual isotopic compositions of particulate nitrogen (PN). The delta(15)N(PN) values ranging from -16.4 per thousand to 3.8 per thousand lie in the lower end of nitrogen isotopic compositions (-16.4 to +18.7 per thousand) of suspended particulate matter observed in estuaries, while the delta(13)C values of particulate organic carbon (POC) and the C/N (organic carbon to nitrogen) ratios showed rather normal ranges from -25.5 per thousand to -19.0 per thousand and from 6.0 to 11.3, respectively. There were three major types of particulate organic matter (POM) in the estuary: natural terrigenous materials consisting mainly of soils and bedrock-derived sediments, anthropogenic wastes and autochthonous materials from the aquatic system. During the typhoon induced flood period in August 2000, the flux-weighted mean of delta(13)C(POC) values was -24.4 per thousand, that of delta(15)N(PN) values was +2.3 per thousand and that of C/N ratio was 9.3. During non-typhoon periods, the concentration-weighted mean was -23.6 per thousand for delta(13)C(POC), -2.6 per thousand for delta(15)N(PN) and 8.0 for C/N ratio. From the distribution of delta(15)N(PN) values of highly polluted estuarine waters, we identified the waste-dominated samples and calculated their mean properties: delta(13)C(POC) value of -23.6+/-0.7 per thousand, delta(15)N(PN) value of -3.0+/-0.1 per thousand and C/N ratio of 8.0+/-1.4. Using a three end-member mixing model based on delta(15)N(PN) values and C/N ratios, we calculated contributions of the three major allochthonous sources of POC, namely, wastes, soils and bedrock-derived sediments, to the estuary. Their contributions were, respectively, 83%, 12% and 5% under non-typhoon conditions, and 9%, 63% and 28% under typhoon conditions. The autochthonous POM had the most varied isotopic compositions, encompassing the full ranges of delta(13)C(POC) (-25.5 to -19.1 per thousand), delta(15)N(PN) (-16.4-3.8 per thousand) and C/N ratio (6.0-11.3). The heavy end of the carbon isotopic composition reflected the typical marine condition and the lower end the estuarine condition, which probably had elevated concentrations of dissolved inorganic carbon with low delta(13)C values due to input from decomposition of organic matter. The lack of isotopically heavy PN, as found in larger estuaries, was attributed to isotopically light starting materials, namely, anthropogenic wastes, the slow phytoplankton growth within the estuary and the rather short residence time; the latter two factors made (15)N enrichment during ammonia consumption very limited. The most isotopically light PN likely originated from phytoplankton incorporating (15)N-depleted nitrate near the river mouth, where ammonia inhibition of nitrate uptake probably stopped.